Fuel blocks, fuel elements, fuel assemblies, and other structures that are used in nuclear reactor cores are generally formed by extrusion or sintering processes. Such processes include preparation of powders that will form the structures and formation of a die through which the powder will be extruded or in which the powder will be compacted and sintered.
After powder preparation, extrusion includes mixing the powders with a binder material, a resin, a lubricant, or combinations thereof and extruding the mixture through a die having a cross-sectional shape of a final structure. The extruded product (i.e., the extrudate) may then be machined to a desired final size and shape.
Alternatively, structures may be formed by compacting a powder mixture into a mold or die and sintering the powder mixture to form a final structure. Such sintering processes often include adding a resin material, a binder material, or a combination thereof to the powder mixture to facilitate formation of a green structure to be sintered. The resin or binder material may comprise, for example, a high molecular weight material, such as a high molecular weight poly(ethylene oxide), a coal tar pitch (CTP) material, other binder materials, or combinations thereof. After the materials are pressed in the mold, the materials are generally heated under high-temperature and high-pressure (HTHP) conditions to form inter-granular bonds between individual grains of the powder material.
Unfortunately, the resin materials and/or binder materials used in sintering or extrusion processes are often undesirable in the final structure. For example, resin or binder materials in an extrudate may increase a likelihood of surface cracking and internal cracking of the extrudate. Resin and binder materials are frequently outgassed from a sintered structure. However, outgassing of the resin or binder material may form pores in the sintered structure, reducing a total density of the final structure. In some instances, outgassing may also cause cracks in the structure. In addition, the resin and binder materials may also reduce a purity of the final structure.